Various arrangements have heretofore been proposed for anchoring a drill ship during drilling operations at sea. One arrangement heretofore proposed utilizes a rotating turret from which a plurality of anchor cables extend to widely spaced anchor points on the ocean floor. The turret is rotatable with respect to the ship so that the turret can be held by the anchors over a drilling location while the heading of the ship can be changed by rotating the vessel relative to the turret. By carrying on drilling through the center of the turret, the heading of the ship can be changed without interrupting the drilling operations. The turret type mooring system is described, for example, in U.S. Pat. Nos. 3,191,201 and 3,279,404.
To provide adequate mooring, it is general practice to use as many as eight anchors spaced in a wide circle around the drill site on the ocean floor. Cables from each of the anchors are brought up into the turret and directed onto suitable winches having drums on which the respective cables are wound. The winches and associated drums must be mounted on the turret structure so as to remain in fixed relation to the turret while allowing the ship to rotate freely about the axis of the turret. To provide room for the winches and cable storing drums, the turret structure is extended to the main deck, the top of the turret being provided with a large platform at the level of the main deck which rotates with the turret and on which the winches and cables drums are mounted. This arrangement has the disadvantages that it occupies valuable space on the main deck, interferes with movement of equipment (BOP stacks, etc.) into the center well (moonpool), and requires that the drilling platform be raised to provide necessary clearance to the main deck. This latter in turn can adversely affect the transverse stability of the vessel by raising the center of gravity relative to center of buoyancy.